Raw onion's double threat to cats
That stolen chunk poses two risks - the onion itself plus potential choking hazards from the fibrous layers. Cats lack the liver enzymes to process onion's sulfur compounds, making them even more vulnerable than dogs. At 4kg, your cat ingested a significant percentage of the toxic dose in one gulp.
Unlike cooked onion which softens, raw onion retains all its harsh juices that irritate the mouth and stomach lining immediately. You might notice excessive drooling or lip-licking within the hour as these local effects appear, separate from the blood cell damage coming later.
The silent countdown has started
Right now, your cat seems fine because the real damage happens invisibly. Onion compounds bind to hemoglobin, creating Heinz bodies that spleen filters will later remove. This process takes 1-5 days, during which red blood cell counts drop steadily without obvious symptoms.
First signs often get missed - slight lethargy, less interest in high places, or taking extra naps. These subtle changes precede the crisis phase where gums turn white, breathing accelerates at rest, and the cat collapses from oxygen starvation. By then, emergency care becomes critical.
Why holiday onions are extra risky
Buffet onions are typically larger and stronger than grocery store varieties - think Bermuda onions versus standard yellows. Holiday cooking often uses sweet onions which cats find more palatable, increasing how much they'll voluntarily eat before the bitter aftertaste stops them.
Your cat didn't just steal any onion piece - it likely chose the juiciest, most aromatic section from the center where sulfur compounds concentrate. Unlike measured pet food ingredients, you have no way to estimate exact weight consumed, which demands assuming worst-case amounts.
Action steps while waiting for symptoms
Collect any remaining onion pieces to show the vet - they can compare size to what's missing. Confine your cat to one room with white bedding to easily spot vomit or dark urine. Offer water frequently but don't force it - hydration helps but stress worsens absorption.
Schedule a vet visit now rather than waiting - baseline bloodwork today allows comparison when symptoms appear. Ask about vitamin E and N-acetylcysteine supplements that may support red blood cell membranes. Never give these without veterinary guidance as dosing is critical for cats.
The full verdict, portion guidance and serving notes are collected on the Onion food facts page.